The present invention relates to a semiconductor apparatus, and more particularly to a semiconductor apparatus employing bonding technology.
In recent years, a semiconductor apparatus equipped with a semiconductor element such as an IGBT (insulated gate bipolar transistor) has been used as a power converter such as an industrial inverter (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2005-116702).
FIG. 8 is a sectional view schematically showing the essential parts of a conventional semiconductor apparatus.
FIG. 8 is useful in explaining the basic construction of the semiconductor apparatus equipped with an IGBT element.
In this semiconductor apparatus, an insulated substrate 202 is bonded to a Cu (copper) base 200, which is a cooling element, via a solder 201. A resin case 205 is fixed to an upper end edge 203 of the Cu base 200 in such a manner as to enclose the IGBT element 204.
An emitter terminal 206 and a collector terminal 207, which are external lead-out terminals, are provided on an inner wall of the resin case 205 and electrically connected to a circuit pattern 210, which is formed on the insulated substrate 202, via aluminum wires 208 and 209, respectively.
A heat spreader 212 is bonded to an emitter electrode, not shown, which is located on an upper surface of the IGBT element 204, via a solder 211. The heat spreader 212 is a metallic plate that spreads heat generated during operation of the IGBT element 204 and is electrically connected to the emitter electrode formed on the surface of the IGBT element 204. One end of an aluminum wire 203 is bonded to an upper surface of the heat spreader 212, and the other end of the aluminum wire 203 is connected to the circuit pattern 210 formed on the insulated substrate 202. That is, the emitter electrode on the IGBT element 204 and the emitter terminal 206 are electrically connected to each other via the aluminum wires 208 and 213.
Also, a collector electrode, not shown, is formed on a lower surface of the IGBT element 204 and connected to the circuit pattern 210 on the insulated substrate 202 via a solder 214. Also, a gate electrode, not shown, is formed on the surface of the IGBT element 204, and an aluminum wire 215a for electrically connecting the gate electrode and the circuit pattern 210 with each other is bonded to the IGBT element 204. An aluminum wire 215b extends from the part of the circuit pattern 210 which is electrically connected to the gate electrode, and an end of the aluminum wire 215b is connected to a gate terminal, not shown, provided inside the resin case 205.
As described above, in a semiconductor apparatus equipped with an IGBT element or the like, components are bonded to one another using metallic wires so as to ensure electrical connection between the components.
Also, recently, there has been proposed a method in which a metallic conductor plate is used in place of the aluminum wires 208, 209, and 213 illustrated in FIG. 8 (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2004-965135).
It has been reported that according to this method, heat dissipation can be improved as compared with a case where aluminum wires are used since a flat-shaped conductor plate is bonded onto an electrode of a semiconductor element.
In this reported example, bonding of the conductor plate is carried out using an ultrasonic bonding method.
However, in e.g. the semiconductor apparatus illustrated in FIG. 8, if bonding of a conductor plate in place of the aluminum wires 208, 209, and 213 is carried out by ultrasonic bonding, a crack may appear in the solder 211 at the bonding interface between the IGBT element 204 and the heat spreader 212 due to vibrations and loads during welding. As a consequence, adequate continuity between the emitter electrode and the heat spreader 212 may not be ensured. Therefore, it is preferred that bonding using a conductor plate in place of wires is carried out by laser welding.
However, if a conductor plate is bonded onto the heat spreader 212 by laser welding, the bonding interface between the conductor plate and the heat spreader 212 is locally heated by laser irradiation. The locally generated heat is transmitted to a semiconductor element such as the IGBT element 214 through the heat spreader 212, which causes heat damages to the semiconductor element.
The present invention has been developed in view of the above described circumstances and provides a semiconductor apparatus which can reduce heat damage even if laser welding is carried out.
Further objects and advantages of the invention will be apparent from the following description of the invention.